This invention relates to the treatment of aqueous hydrofluoric acid solutions of niobium emanating from the liquid-liquid extraction and purification of niobium.
The usual purification and recovery of niobium from its ores involves taking the ore into hydrofluoric acid solution and separating the niobium from tantalum and gangue materials by its differential solubility in organic extractants such as methyl-isobutyl-ketone, di-alkyl-phosphate, tributyl-phosphate, trioctyl-amine and the like. The niobium fluoride loaded organic phase is then contacted with water whereby the niobium values are preferentially soluble in the aqueous phase. This aqueous niobium fluoride solution is then treated with ammonia to precipitate the niobium as the hydroxide along with chemisorbed ammonium fluoride. This slurry is then filtered thus separating the ammonium fluoride solution from the precipitated niobium hydroxide. The niobium hydroxide filter cake is then dried and calcined at 875.degree. to 1000.degree. C. to remove moisture and absorbed ammonium fluoride. The ammonium fluoride solutions create a waste disposal problem and must be treated with lime to react with the ammonium fluoride according to the reaction: EQU 2NH.sub.4 F+ CaO.fwdarw.CaF.sub. 2 + 2NH.sub.3 + H.sub.2 O (I)
generally, this reaction takes place in an ammonia stripping column whereby the released ammonia is recovered in the overhead fraction.
The calcined Nb.sub.2 O.sub.5 can then be alumino-thermically reduced in the presence of an alloying element such as iron, nickel, or chromium to produce a vacuum-grade niobium-containing alloy.
It has now been found that by recovering niobium values from aqueous hydrofluoric acid strip solutions which are evaporated to dryness following which the residue is baked at a temperature from about 150.degree. C. to 250.degree. C., niobium oxyfluoride (NbO.sub.2 F) is recovered which can be alumino-thermically reduced to a vacuum-grade alloy. The process is performed in a simple efficient manner and avoids numerous disadvantages of the prior art. In addition, about 1.5 pounds hydrogen fluoride per pound of niobium is recovered in the process.